Hello!

Your Edge 540 was built for 60-90 size engines. The 91FX has 560g without muffler, the 120 AX about 647gramms.
It woud cause CG problems. But that can be fixed with the right accu and servo positioning.
But at 9 pounds the 91FX (without tuned pipe) would be not enough for really hard 3D.

So I think there is NO "too much" if U take the 1.20 !